Enable scheduling new tasks such that previous tasks are not unscheduled

[krashish8]

Fixes #15, #22

Changes proposed in this pull request:

• Fresh scheduling (fresh=true query parameter in Schedule POST API endpoint):
  • Schedule request with dropping all previous allocations
• Normal scheduling - DEFAULT (fresh=false query parameter in Schedule POST API endpoint):
  • Schedule request such that previous tasks are not unscheduled, and are shifted by the "max_shift" interval
  • "max_shift" column defined in the projects, with default value of '00:30:00'::INTERVAL (30 mins).

[codecov-commenter]

Codecov Report

Merging #31 (3850fcd) into develop (eec5cb6) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #31      +/-   ##
===========================================
+ Coverage    76.10%   76.12%   +0.02%     
===========================================
  Files           36       36              
  Lines         2352     2354       +2     
===========================================
+ Hits          1790     1792       +2     
  Misses         414      414              
  Partials       148      148              

Impacted Files	Coverage Δ
e2e_test/util.go	48.27% <100.00%> (+3.83%)	:arrow_up:
internal/database/schedule.go	84.54% <100.00%> (ø)

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